Is Altruism Dead?

I was doing errands and remembered to drop of the mail. There's a drive-by mailbox that I frequent. A car was parked in front of the boxes, so I parked behind him, found my mail, got out, locked the doors (because I leave my son in there for the 10 seconds and I couldn't imagine leaving the doors unlocked), and started walking towards the mailbox. Then, the guy who was just ahead of me asked "would you like me to take that?" I thought about it, accepted, and thanked him. I'm a pretty paranoid person, so I watched him out of the corner of my eye as he walked over and tossed the mail in. I didn't have a return address label on this particular piece of mail, so that wasn't an issue. And I've since checked to make sure the mail arrived. We both got in our respective cars and drove away.

And then I realized, maybe he's just nice! There was no additional effort on his part to take the mail over than to stop and ask me. But as a jaded Angeleno or American or whatever...I am trained to distrust and watch for the "angle." Perhaps he was looking for something...address, etc. But the funny thing is, I'm a pretty honest person and if I were to offer the same thing to a stranger, it would be completely altruistic. So why do I have such a hard time believing that someone else is capable of the same thing?